Hydromorphology as a controlling factor of macrophytes assemblage structure and functional traits in the semi-arid European Mediterranean streams

Abstract

Macrophytes have a crucial impact on stream functioning. However, there is a significant gap of knowledge about how hydromorphological fluctuations affect their structural and functional responses in southern Mediterranean streams. In this study, we investigated the impact of hydromorphology on macrophyte stream assemblages in Cyprus and analysed their structural and functional responses. We collected macrophytes and hydromorphological data from 63 sites along a gradient from permanent to intermittent streams. We applied Principal Component Analysis (PCA) to identify and characterise stream sub-types. We performed an Indicator Species Analysis (ISA) and estimated taxonomical diversity indices to investigate whether differences among stream sub-types affect macrophytes assemblage structure. Functional responses to the flow regime were tested by allocating traits related to persistence, regeneration, dispersibility and ecological preferences for moisture, light, nutrients and salinity. The results indicated the existence of two permanent and two intermittent flow sub-types. A total of 25 indicator species were identified showing taxonomic variation in macrophyte assemblages among streams with different flow regimes. We demonstrated that flow intermittency promotes a higher number of indicator species with wider ecological preferences and traits allowing resilience to drought. Specifically, we found that macrophytes in the intermittent streams, survive during dry period through the establishment of dormant seed bank or through belowground organs. They also showed lower values of Specific Leaf Area and therefore a reduction of water loss through evapotranspiration. In the light of climate change, where droughts are anticipated to increase, more permanent streams will become intermittent especially in south Mediterranean countries, and new habitats will be released, including marginal zones. Our results showed that these alterations in stream hydromorphology will produce changes in macrophyte assemblages which might cause shifts in stream ecosystem functions and services. Therefore, our knowledge about the direction of these changes is crucial for future management and conservation plans.

Keywords: Cyprus; Functional and taxonomic diversity indices; Macrophytes ecological preferences; Perennial and intermittent streams; Stream flow regime.